1. Field of Invention
The present invention relates generally to golf clubs and, more particularly, to golf club shafts.
2. Description of the Related Art
Over the years, many substitutes have been introduced for the hard wood shafts originally used in golf club drivers and irons. Early substitute materials included stainless steel and aluminum. More recently, carbon fiber reinforced resin shafts have become popular. Fiber reinforced resin shafts are typically hollow and consist of a shaft wall formed around a tapered mandrel. The mandrel typically consists of three mandrel sections. The first mandrel section forms the tip section of the shaft, the second mandrel section forms the main body section, and the third mandrel section forms the grip section. As shown in FIG. 1, shafts formed in this manner typically have a constant taper from the tip/main body intersection to the main body/grip intersection. In other words, the taper of the main body section is constant. Additionally, in order to reduce the weight of the shaft, the shaft wall thickness in conventional shafts tends to decrease uniformly (i.e. at a constant rate without abrupt changes), at least from the tip/main body intersection to the main body/grip intersection.
The use of fiber reinforced resin has allowed golf club manufacturers to produce shafts having varying degrees of torsional and longitudinal stiffness to satisfy the needs of a wide variety of golfers. Torsional stiffness relates to a golf club""s ability to resist twisting along its length when a golf ball is struck. The inertia of the ball produces a force on the head tending to rotate the head about the axis of the shaft relative to the grip section. Longitudinal stiffness refers to a golf club""s ability to resist bending when subjected to a force.
For a given grip outer diameter (OD) and a given tip OD, the conventional method of increasing the torsional and longitudinal stiffness of a fiber reinforced resin shaft is to increase the thickness of the shaft wall. However, because the fiber reinforced resins used to make the shaft are expensive, the use of additional material to increase the shaft wall thickness raises the cost of the shaft to an undesirable level. Additionally, increasing the shaft wall thickness adds weight to the shaft, which is also undesirable. Another method of increasing torsional and longitudinal stiffness is to use materials with a higher modulous of stiffness. Due to the higher cost of these materials, this method is also undesirable.
More recently, so-called xe2x80x9cbubblexe2x80x9d or xe2x80x9cwide bodyxe2x80x9d shafts have been introduced in an attempt to increase shaft stiffness. Here, the OD of selected portions of the main body section is greater than that of a shaft having a constant taper from the tip section to grip section. In some cases, the main body section""s proximal portion (the portion of the main body section closest to the golfer) will have a greater OD than the distal end of the grip section (the end of the grip section farthest from the golfer). As such, the main body section of certain xe2x80x9cbubblexe2x80x9d or xe2x80x9cwide bodyxe2x80x9d shafts includes a short connecting portion which rapidly decreases in diameter, thereby connecting the main body section to the grip section. Such a shaft is disclosed in U.S. Pat. No. 5,316,299 to Feche et al. The shaft stiffness is generally greater in xe2x80x9cbubblexe2x80x9d or xe2x80x9cwide bodyxe2x80x9d shafts than in conventional shafts formed with the same materials and having the same wall thickness. Nevertheless, there are limitations with respect to the maximum OD of the main body section. Accordingly, there is a need for additional methods of increasing shaft stiffness that do not substantially increase the weight or cost of the shaft.
Accordingly, the general object of the present invention is to provide a golf club shaft which avoids, for practical purposes, the aforementioned problems. In particular, one object of the present invention is to provide a golf club shaft which has greater longitudinal and torsional stiffness than conventional shafts with the same tip OD and grip OD. Another object of the present invention is to provide a golf club shaft which has greater longitudinal and torsional stiffness than conventional shafts with the same tip OD and grip OD without substantially increasing the cost of the shaft. Still another object of the present invention is to provide a method of manufacturing golf club shafts which allows the longitudinal and torsional stiffness to be easily varied for a given tip OD and grip OD without substantially varying the weight of the shaft.
In order to accomplish these and other objectives, a golf club shaft in accordance with one embodiment of the present invention has a stiffness control section between the grip section and the main body section. The stiffness control section includes a first tapered portion and a second tapered portion. The circumference of the first tapered portion increases from the proximal end to the distal end thereof and the circumference of the second tapered portion decreases from the proximal end to the distal end thereof.
The present invention provides a number of advantages over the prior art. For example, a shaft with the present stiffness control section has greater longitudinal and torsional stiffness than a shaft with a continuous taper or conventional shafts with the same tip OD and grip OD. Moreover, the present invention does so without increasing the cost or weight of the shaft. Alternatively, the present invention may be used to provide a shaft that, although lighter in weight, has the same stiffness, tip OD and grip OD as a conventional shaft. The present invention also provides a number of advantages over xe2x80x9cbubblexe2x80x9d shafts. In order to increase the stiffness of a xe2x80x9cbubblexe2x80x9d shaft, the size of the bubble portion is increased, which can adversely effect the appearance of the shaft. On the other hand, increasing stiffness through the use of conical sections, as in the present invention, provides more gradual diametric changes and a less abrupt appearance. Additionally, adjusting the length of the present stiffness control section allows the shaft""s point of maximum OD to be moved towards the tip of the shaft, thereby increasing the stiffness of the shaft and lowering torque. Such adjustments are not possible with xe2x80x9cbubblexe2x80x9d shafts.
The above described and many other features and attendant advantages of the present invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.